Fire resistant hydraulic fluids and lubricants



United States Patent 3,383,318 FIRE RESISTANT HYDRAULIC FLUIDS ANDLUBRECANTS Kenneth L. McHugh, Kirlrwood, and Kurt A. Nowotny,

Rock Hill, Mo., assignors to Monsanto Company, St. Louis, Mo., acorporation of Delaware No Drawing. Filed Mar. 31, 1965, Ser. No.444,466 7 Claims. (Cl. 252-78) ABSTRACT OF THE DISCLOSURE Compositionsuseful as functional fluids comprising (1) a mixture of compoundsrepresented by the structural formula wherein R and R are each alkylradicals having from 3 to 6 carbon atoms, X is selected from the groupconsisting of hydrogen, alkyl and alkoxy radicals having from 1 to 8carbon atoms, and the halogens, and (2) compositions comprising (a) amajor amount of a compound represented by the structural formula whereinY and Z are each selected from the group consisting of oxygen andsulfur, R and R are each alkyl radicals having from 3 to 6 carbon atoms,X is selected from the group consisting of hydrogen, alkyl and alkoxyradicals having from 1 to 8 carbon atoms, and the halogens, a, b and care integers from 0 to 3 and the sum I of a, b and c is 3, and mixturesthereof.

This invention relates to novel functional fluids comprising certainesters of phosphinic acid.

Many different types of materials are utilized as functional fluids andfunctional fluids are used in many different types of applications. Suchfluids have been used as electronic coolants, atomic reactor coolants,diffusion pump fluids, synthetic lubricants, clamping fluids, bases forgreases, force transmission fluids (hydraulic fluids) and as filtermediums for air conditioning systems. Because of the wide variety ofapplications and the varied conditions under which functional fluids areutilized, the properties desired in a good functional fluid necessarilyvary with the particular application in which it is to be utilized. Eachindividual application requires a functional fluid having a specificclass of properties.

Of the foregoing, the use of functional fluids as hydraulic 7 fluids,particularly aircraft hydraulic fluids, has posed what is probably themost ditficult area of application.

3,383,318 Patented May 14, 1968 Thus, up to a few years ago therequirements for an aircraft hydraulic fluid could be described asfollows:

The hydraulic power systems of aircraft for operating various mechanismsof an airplane impose stringent requirernents on the hydraulic fluidused. Not only must the hydraulic fluid for aircraft meet stringentfunctional and use requirements but in addition such fluid should be ashighly non-flammable as possible and must be sufliciently non-flammableto satisfy aircraft requirements for fire resistance. The viscositycharacteristics of the fluid must be such that it may be used over awide temperature range; that is, adequately high viscosity at hightemperature, low viscosity at low temperature and a low rate of changeof viscosity with temperature. Such temperature range is generally from40 F. to 450 F Its pour point should be low. Its volatility should below at elevated temperatures of use and the volatility should bebalanced; that is, selective evaporation or volatilization of anyimportant component should not take place at the high temperatures ofuse. It must possess sufilcient lubricity and mechanical stability toenable it to be used in the selflubricated pumps, valves, etc. employedin the hydraulic systems of aircraft which are exceedingly severe on thefluid used. It should be thermally and chemically stable in order toresist oxidation and decomposition so that it will remain uniform underconditions of use and be able to resist the loss of desiredcharacteristics due to high and sudden changes of pressure andtemperature, high shearing stresses, and contact with various metalswhich may be, for example, aluminum, bronze, copper and steel. It shouldalso not deteriorate the gaskets or packings of the hydraulic system. Itmust not adversely aflect the materials of which the system isconstructed, and in the event of a leak, should not adversely affect thevarious parts of the airplane with which it may accidentally come incontact, such as electrical wire insulation and paint. It should not betoxic or harmful to personnel who may come in contact with it.

While the above-stated requirements are stringent, the development ofindustry in general and air transportation in particular has renderedthese old requirements and specifications inadequate for present andfuture needs. Industrial automation has increased the importance of fireresistance in hydraulic fluids. The air transportation industry has beendeveloping new concepts of aircraft design at a rapid pace. Theincreased size and speed of aircrafts have made necessary extendedhorsepower demands for aircraft motive power units and associatedhydraulic control systems thus increasing working temperature ranges andoperating pressures. As the speed of commercial aircraft approach andsurpass that of sound, operating temperatures are incurred which requirethe designers to take advantage of all heat dissipating techniquesavailable in the craft. Surface temperatures of a Mach 3 aircraft willrange from 450 F. to 600 F. or higher at stagnation points. By takingadvantage of natural heat sinks, such as the fuel in a manner utilizedin the B70, the hydraulic system should be capable of performing with afluid operating at 400 F. to 500 P. On the other end of the temperaturescale, temperatures as low as 40 F. are anticipated.

0 seal-s. This leakage problem was considered by the panel and industryin general to be a very undesirable situation from the standpoint ofloss of powered control. In the supersonic aircraft any leakage problemswould be magnified excessively over and above the loss of poweredcontrol when one considers the temperatures involved. In this case,there is no longer the situation in which leakage fluid will issue intorelatively cold areas but rather into ambient temperatures as high as 60F. It is apparent that a flammable fluid injected into hot compartmentswould create a blow torch effect, an untenable condition. A fireresistant fluid is thus of greater importance than ever before.

The principal problem facing a fiuid supplier, therefore, is that ofdeveloping a hydraulic fluid having temperature compatibility toapproximately 400 F. to 500 F. combined with fire resistance. Inaddition to the foregoing the hydraulic fluid must still have theproperties mentioned above, including good viscosity characteristics(over a quite extended temperature range), a low freezing point, lowvolatility, sufficient lubricity, no toxicity and compatibility withvarious metals, packings and gaskets.

It is, therefore, an object of this invention to provide functionalfluid compositions having a combination of properties, such as wideliquid range and fire-resistance, which makes such compositions wellsuited for the various applications mentioned above. It is a furtherobject of this invention to provide functional fluid compositions whichare useful as hydraulic fluids, particularly aircraft hydraulic fluids.A further object is to provide functional fluids useful as hydraulicfluids in supersonic aircraft. Other objects will be apparent from thefollowing description of the invention.

It has now been found that functional fluids having excellentfire-resistance coupled with the physical properties necessary toprovide compositions useful for the many applications disclosed aboveand particularly as aircraft hydraulic fluids comprise certain aryldialkyl phosphinate esters which can be represented by the structuralformula:

I. Y R;

wherein Y and Z are each selected from the group consisting of oxygenand sulfur, R and R are each alkyl radicals having from 3 to 6 carbonatoms, X is selected from the group consisting of hydrogen, alkyl andalkoxy radicals having from 1 to 8 carbon atoms, and the halogens.Functional fluids comprising mixtures of compounds represented bystructure I are also intended to be within the scope of this invention.

Also within the broad aspects of this invention, there is provided novelfunctional fluids comprising a compound or mixture of compounds ofstructure I above in admixture with blending agents or mixtures ofblending agents which can be represented by the structural formula II. Y(Ri)b 4 phenyl-(n-butyl-n-pentyl) phosphinate, phenyl-(n-butyl-n-hexyl)phosphinate, phenyl-(n-pentyl-n-hexyl) phosphinate,phenyl-(neopentyl-n-propyl) phosphinate, phenyl-(neopentyl-n-butyl)phosphinate, phenyl-(neopentyl-n-hexyl) phosphinate,thiophenyl-di-n-propyl phosphinate, thiophenyl-di-n-pentyl phosphinate,cresyl-di-n-pentyl phosphinate terL-butylphenyl-di-n-butyl phosphinate,n-butylphenyl-di-n-butyl phosphinate, secmbutylphenyl-di-n butylphosphinate, ethylphenyl-di-mbutyl phosphinate, xylyl-di-n-butylphosphinate, thiophenyl-di-n-hexyl phosphinate, thiophenyl-di-n-butylphosphinate, thiophenyl-di-n-propyl thiophosphinate,thiophenyl-di-n-butyl thiophosphinate, thiophenyl-di-n-pentylthiophosphinate, thiophenyl-di-n-hexyl thiophosphinate,thiophenyl-(n-propyl-n-butyl) phosphinate,thiophenyl-(n-propyl-n-pentyl) phosphinate,thiophenyl-(n-propyl-n-hexyl) phosphinate, thiophenyl-(n-butyl-n-pentyl)phosphinate, thiophenyl-(n-butyl-n-hexyl) phosphinate,thiophenyl-(n-pentyl-n-hexyl) phosphinate, thiophenyl-(n-propyl-n-butyl)thiophosphinate, thiophenyl-(n-propyl-n-pentyl) thiophosphinate,thiophenyl-(npropyl-n-hexyl) thiophosphinate,thiophenyl(n-butyl-n-pentyl) thiophosphinate, thiophenyl(n-butyl-n-hexyl) thiophosphinate, and thiophenyl-(n-pentyl-n-hexyl)thiophosphinate.

Typical examples of the compounds of structure II are as follows:

tris-n-butyl phosphine oxide, tris-n-p'ropyl phosphine oxide,tris-n-pentyl phosphine oxide, tris-n-hexyl phosphine oxide,diphenyl-n-propyl phosphonate, diphenyl-n-butyl phosphonate,diphenyl-n-pentyl phosphonate, diphenyl-n-hexyl phosphonate, triphenylphosphate, and tricresyl phosphate.

As used herein the term major amount of a base stock means that theamount of a particular base stock in :a specific formulation is at leastequal to the amount of any particular blending agent in saidformulation. On the other hand the term minor amount of a blending agentmeans that the amount of a particular blending agent in a specificformulation is no more than the amount of any specific base stock insaid formulation.

Although the compounds of structure I are useful per se as functionalfluids, it is preferred to employ said esters as the major amount of acomposition wherein a compound of structure II or a mixture of suchcompounds are present in minor amounts. Such compositions preferablycontain from about 60% to about of a compound of structure I above.Specifically, a preferred composition of this invention contains fromabout 60% to about 85 aryl dialkyl phosphinate, from about 5% to about20% trialkyl phosphine oxide and from about 5% to about 20% diaryl alkylphosphonate.

The above-mentioned blended compositions can be prepared by simplyadding specific quantities of compounds of structure 11 to a phosphinicester of structure I. It has been discovered, however, that compositionsof the type herein described can be prepared directly by reacting anaryl phosphorochloridate, represented by the structure wherein R isaryl, Y and Z are selected from the group consisting of oxygen andsulfur and m and n are integers from 1 to 2 and the Sum of m+n is 3,with Grignard reagents if (a) the Grignard reagent -is of the type R MX,

6 Example 4 In the manner of Example 1, 785.5 parts of n-pentylmagnesiumchloride in ether is added to 633 parts of phenylphosphorodichloridatein ether. The reaction prod- Where is y M 18 magileslum alummum orlithlum 5 uct is cooled, quenched and washed yielding a composiand X ischlorine or bromme, and (b) such Grignard tion containing by Weight 2%O-phenyl di-n-pentyl reagent 18 added the aryl Ph mP T phosphinate, 9%tris-n-pentyl phosphine oxide and 9% reaction product 18 a composltioncontaining a ma or diphenyl n pentyl phosphonate amountof compound ofsvtrhlcture'l and mmor amounts The individual phosphinic acid esters ofstructure I f compounds of structure i P the Hinge of respec' 10produced according to the procedures illustrated in the Preferred,amounts mdlcated above' above example can be easily obtained in pureform by followmg examples lllustratF the methcfd 'f 'f fractionaldistillation of the reaction product under reing directly the preferredcompositions of this invention, duced pressure in which parts are partsby weight and the reactor used Typical Propel-ties of the above preparedand other 1s a conventional glass reactor fitted with an agitator, abase Stock compounds of this invention are set forth in refluxcondenser, raw material inlet, product outlet, heat- Table 1, below Themethods of obtaining these data are mg means and thermometer. asfollows;

The melting point of pure compounds or solution points Examplel of thecompositions of this invention were measured. Because the compositionsof the instant invention easily A reactor as heretofore described ischarged with 422 superfool (as do components) E Q P parts ofphenylphosph-orodichloridate in 5 00 parts of are dflficult}? deterPnnefPomt ether. With the agitator running 6454 Parts of and crystalllzmgpoint coincide the solution pomt was magnesium bromide is slowly(-dropwise) added over a generally measured period of one hour. Anexothermic reaction is noted Solution Points Were determined y placing atest cornand the reaction mixture is cooled to room temperature.Position in a Test tube Provided With an agitator and The reactionmixture is then acidified With10% sulfuric suspending the apparatus in awell insulated y :acid and the resulting ether layer separated. Th th racetone bath. The Dry Ice-acetone bath was maintained layer is thenwashed with 10% sulfuric acid followed by 3 at a temPefaPuTe in therange of tO additional washing with 10% sodium hydroxide. Washa rangeconsldel'ed high enough to Prevent a glass from ing is repeated withwater until the washings are neutral. forming and low enough to Speed pPotential y The ether is then evaporatfld yielding a compositionlizat1on. After a test composition had been agitated for mining byWeight 0% pheny1 dibutyl phosphinate, about eight hours, seeds of one ofthe components were 20% trisqlabutyl phosphine oxide and 2 diphenyl nadded. The seeded composition was then stored n a (fold butylphosphonate, bOX at F. for sixteen hours and then agitated in the DryIce-acetone bath for eight hours. The cycle was then repeated. Thosemixtures which did not crystallize Example 2 after one week were warmedto room temperature and In the manner of Example 1, 4675 parts of nbuty1 0 transferred to small bottles with lids. The bottles weremagnesium chloride in ether is added to 422 parts of then Placed In Coldstorage phenylphosphorodichloridate in ether. The reaction prod- Thethermal Stability of the components and p uct is cooled, quenched andwashed yielding a compositions of this invention were determined by theuse of an tion containing by weight O-phenyl di-n-butyl phosisoteniscopeaccording to the procedure of Blake et al., phinate, 10% tris-n-butylphosphine oxide and 10% di- 45 Chema, 87 The basis for this h p b t l hh t procedure is that when a fluid is heated in the isoteniscopeapparatus, it exerts a vapor pressure which can be readily Ex 16 3measured. The vapor pressure increases as temperature amp is mcreasedfollowing a straight-line relationship with In the manner of Example 1,289.7 parts of n-hexyl- 50 logarithm of pressure is plotted versus thereciprocal of magnesium chloride in ether is added to 211.0 parts of theabsolute temperature. The vapor pressure curve willphenylphosphorodichloridate in ether. The reaction proddepart from astraight line if decomposition occurs to not is cooled, quenched andwashed yielding a composigive volatile products. The temperature atwhich this oction containing by weight O-phenyl di-n-hexyl phoscurs iscalled the decomposition temperature (T phinate, 7.5% tris-n-hexylphosphine oxide and 7.5% Viscosity measurements were conducted undercondidiphenyl-n-hexyl phosphonate. tions set forth in ASTM 0-445-61.

TABLE I Solution Boiling Thermal Viscosity Compound Point, F. Point, F./Stability mm. pressure (To), F. -40 F F. 210 F Phenyl-di-n-hexylphosphinate 20 155/0. 2 528 5, 520 12. 58 2. 63 Phenyl-di-n-pentylphosphinate 10 162/0. 5 534 53 413? 10.70 2. 31 Phenyl-di-neopentylphospinate 50 152/08 547 10%%(i3 18.62 3.01 Phenyl-di-n-butylphosphinate 80 136/0. 4 633 4, ass 9. 02 2. 11 Phenyl-di-sec-butylphosphinate 45 123/01 586 10.03 2. 08 Phenyl-n-propyl-n-pentylph0spl1inate 85 140-146/0. 15 530 3,852 8. 94 2.08 Phenyl-di-n-propylphosphiuate 85 1 15-1 17/10 599 4, 301 7. 43 1. 83p-Methoxyphenyl-di-n-butyl phosphinate 164/0. 3 564p-Nitrophenyl-di-n-butyl phosphinate 182/0. 2 536 Thiophcnyl-di-n-butylthio-phosphinate 40 129/0. 25 514 (OZ, 5:1? 29. 51 3. 30

1 Orystallizing point. 2 crystallized at -10 F.

From the properties set forth in Table I above, it is evident thatphosphinate ester base stock compounds have a combination of physicalproperties which make them well suited for use as functional fluids.These desirable physical properties are enhanced or improved by blendingthe novel phosphinate esters with minor amounts of compounds ofstructure 11 hereinbefore set forth. To demonstrate the fire-resistantproperty of the compositions of this invention a fluid consisting of 75%phenyldi-n-butyl phosphinate, 12% tris-n-butyl phosphine oxide and 13%diphenyl-n-butyl phosphonate, hereinafter desig nated fluid A, wassubjected to various empirical tests generally recognized as a trueindication of fire resistance. A phosphate ester type fluid,commercially used and accepted as being fire resistant was alsosubjected to the aforementioned empirical tests and is hereinafterdesignated fluid B. The tests or procedures used to measure thefire-resistant properties of the fluid are as follows:

C. Hot manifold test, AMS 3150 High pressure spray test, AMS 3150 Anadditional test often used, which is a smaller scale test, is theMolten-metal Pour Test. In this test the fluid under evaluation isdropped from a medicine dropper or poured from a calibrated test tubeonto the surface of molten aluminum alloy which has been heated to about1250 F. If spontaneous ignition does not occur, a flame is placed in thevapors to determine if they can be ignited. The results of these testsappear in Table II below.

for the absorption and dissipation of energy, such as shock absorbers orrecoil mechanisms or the transmission of torque through torque convertertype fluid couplings. The compositions of this invention can also beused as damping fluids which are the liquid compositions used fordamping mechanical vibrations or resisting other rapid mechanicalmovements. The compositions of this invention are also suitable for useas lubricants between relatively moving metal mechanical parts, as basesfor synthetic greases, as nuclear reactor coolants, as vacuum pumpfluids and as the liquid material in the filtrates of air conditioningsystems. The instant compositions are particularly well suited forcooling and lubricating metal gears and bearings in jet engines.

As a result of the excellent physical properties of the functionalfluids particularly described in the preceding examples, improvedhydraulic pressure devices can be prepared in accordance with thisinvention which comprise in combination a fluid chamber and an actautingfluid in said chamber, said fluid comprising a mixture of one or more ofthe base stocks hereinbefore described. In such a hydraulic apparatuswherein a movable member is actuated by the above-described functionalfluids, performance characteristics are obtainable which are superior tothose heretofore obtainable.

Because of the excellent fire-resistance of the functional fluids ofthis invention, their exceptionally low pour points, and good lubricity,the functional fluids of this invention can be utilized in thosehydraulic systems wherein power TABLE II Compound Molten-metal Pour TestHigh Pressure Spray Test I-Iot Manifold Test Does not ignitespontaneously. Did not ignite up to 4 feet; from orifice. Burned on thetube. Fluid Ignites with spark. Flashed with mild flame beyond 4 feetDid not carry flame from tube.

* from orifice.

Burns to completion-large flame. Flame is not sustaining. Did not burnin pan.

Does not ignite spontaneously. Did not ignite up to 4 f et from orifice.Burned on the tube. d B Ignites with spark. Flashed with mild llanieheyond4 feet Did not carry flame from tube.

" from orifice.

Burns to completion-large flame. Flame is not sustaining. Did not burnin pan Compositions of this invention also possess good lubricating theproperties as evidenced by the results obtained from testing of suchcompositions on the Shell Fourball Wear Test machine. The results arelisted in Table Test Conditions-10 kg. load; 1,200r.p.m. [or '2 hoursat. the temperature indicated.

In addition to the above the compositions of this invention are shearstable and are not prone to foaming and any foam formed is not stable.Furthermore, the claimed compositions have good stability, even attemperatures of 500 F. and in the presence of oxygen, and areessentially non-corrosive to metals, such as iron, silver and titanium.A further advantage of the instant compositions is their outstandinghydrolytic stability.

The compositions of this invention are useful as functional fluids overwide temperature ranges and in various applications, such as for forcetransmission fluids for the transmission of pressure, power or torque influid pressure or torque actuated mechanisms. Specific examples of suchuses are the hydraulic fluids used to transmit fluid pressure to the ramcylinder of hydraulic presses, devices must be transmitted and thefrictional parts of the system lubricated by the hydraulic fluidutilized. Thus, the novel functional fluids of this invention findutility in the transmission of power in a hydraulic system having a pumptherein supplying the power for the system. In such a system, the partswhich are so lubricated include the frictional surfaces of the source ofpower, namely the pump, valves, operating pistons and cylinders, fluidmotors and in some cases, for machine tools, the ways, table and slides.The hydraulic system may be of either the constantvolume or thevariable-volume type of system.

The pumps may be of various types, including the piston-type pump, moreparticularly the variable-stroke piston pump, the variable-discharge orvariable displacement piston pump, radial-piston pump, axial-pistonpump, in which a pivoted cylinder block is adjusted at various angleswith the piston assembly, for example, the Vickers Axial-Piston Pump, orin which the mechanism which drives the pistons is set at an angleadjustable with the cylinder block; gear-type pump, which may be spur,helical or herringbone gears, variations of internal gears, or a screwpump; or vane pumps. The valves may be stop valves, reversing valves,pilot valves, throttling valves, sequence valves or relief valves. Fluidmotors are usually constantor variable-discharge piston pumps caused torotate by the pressure of the hydraulic fluid of the system with thepower supplied by the pump power source. Such a hydraulic motor may beused in connection with a variable-discharge pump to form avariable-speed transmission.

Formulated functional fluids can be prepared by employing compositionsof this invention in admixture with selected additives which are addedin minor proportions to enhance a desirable characteristic or tosuppress an undesirable characteristic. Such additives generally includecorrosion inhibitors or hydrolytic stabilizers such as dibromobenzene,chlorinated biphenyl, alkylated thiophene, triphenylthioether,iodobiphenyl and the like. Such additives can be employed inconcentrations of from about 0.01% to about 20% by weight in thefunctional fluids of this invention.

The compositions of this invention can also contain dyes, pour pointdepressants, antioxidants, viscosity index improvers, such aspolyalkylacrylates and polyalkylmethacrylates, lubricity agents and thelike.

Certain of the aryl dialkyl phosphinate esters of this invention are newand novel. Said esters can be represented by the structural formula III.R1

wherein R and R are each alkyl radicals having from 3 to 6 carbon atoms,X is selected from the group consisting of hydrogen, alkyl and alkoxyradicals having from 1 to 8 carbon atoms and the halogens. Aryl dialkylphosphinate esters wherein the alkyl radicals contain more than 6 carbonatoms have been found -to exhibit undesira'ble characteristics offunctional fluids. It has been found, for example, that aryl diheptylphosphinate and higher homologues of the compounds of structure III arenot useful as base stocks for functional fluids. This lack of utility isattributed to the relatively high viscosity of the compounds at normalroom temperatures and below. In fact the viscosity of these compounds issuch that hydraulic systems containing them as the operative fluid wouldbe inoperative at low temperatures, i.e., 70 F. Such hydraulic systemswould be operative only at elevated temperatures when the operativefluid is, for example, phenyl-di-moctyl phosphinate which is a solid atroom temperature. It has also been found that aryl dialkyl phosphinateesters wherein the alkyl radicals contain less than three carbon atomsexhibit undesirable characteristics of functional fluids in that suchcompounds are highly corrosive to ferrous metals and aluminum. Thus,when the number of carbon atoms in the alkyl portion of the ester isbelow three, the degree of corrosiveness increases surprisingly to aprohibitive level for use as a functional fluid.

While this invention has been described with respect to various specificexamples and embodiments, it is to be understood that the invention isnot limited thereto and that it can be variously practice-d within thescope of the following claims.

The embodiment-s of the invention in which an exclusive property orprivilige is claimed are defined as follows:

1. A composition comprising (a) at least about 60% by weight, of acompound represented by the formula sisting of oxygen and sulfur, R andR are each alkyl radicals having from 3 to 6 carbon atoms, X is selectedfrom the group consisting of hydrogen, alkyl and alkoxy radicals havingfrom 1 to 8 carbon atoms, and the halogens, (b) a minor amount of acompound other than that selected as component (a) selected from thegroup consisting of (l) a compound represented by the formula wherein Yand Z are each selected from the group consisting of oxygen and sulfur,R and R are each alkyl radicals having from 3 to 6 carbon atoms, X isselected from the group consisting of hydrogen, alkyl and alkoxyradicals having from 1 to 8 carbon atoms, and the halogens, a, b and care integers from 0 to 3 and the sum of a, b and c is 3 and (2) mixturesthereof.

2. A composition comprising by weight from about 60% to about 85% phenyldi-n-butyl phosphinate, from about 5% to about 20% tris-n-butylphosphine oxide and from about 5% to about 20% diphenyl-n-butylphosphon-ate.

3. A composition comprising by weight from about 80% to about 99.9% of acomposition of claim 1 and from about 0.1% to about 20% ofdibromobenzene.

4. A composition comprising by weight from about 89% to about 99.9% of acomposition of claim 2 and from about 0.1% to about 20% ofdibromobenzene.

5. The method of operating a hydraulic pressure device wherein adisplacing force is transmitted to a displaceable member by mean-s of ahydraulic fluid, the improvement which comprises employing as saidhydraulic fluid a compound represented by the structural formula:

z X wherein R and R are each alkyl radicals having from 3 to 6 carbonatoms, X is selected from the group consisting of hydrogen, alkyl andalkoxy radicals having from 1 to 8 carbon atoms, and the halogen-s.

6. The method of operating a hydraulic pressure device wherein adisplacing force is transmitted to a displaceable member by means of ahydraulic fluid, the improvement which comprises employing as saidhydraulic fluid a composition of claim 1.

7. The method of operating a hydraulic pressure device wherein adisplacing force is transmitted to a displaceable member by means of ahydraulic fluid, the improvement which comprises employing as saidhydraulic fluid a composition of claim 2.

References Cited UNITED STATES PATENTS 2,629,693 2/1953 Barton et al.252-334 3,287,275 11/1966 Seil 252- OTHER REFERENCES The CondensedChemical Dictionary, Sixth edition, Reinhold Publishing Corporation, NewYork, 1961, p. 356.

LEON D. ROSDOL, Primary Examiner.

S. D. SCHWARTZ, Assistant Examiner.

